1. Field of the invention
The present invention relates generally to an image forming apparatus in which a latent image is developed under pressure, and more particularly to a pressure developing device for use in an image forming apparatus, the device being capable of applying an appropriate pressure to various sheet sizes.
2. Description of the prior art
Japanese Laid-Open Patent Publication No. 63-92954 discloses a pressure developing apparatus having high pressure rollers in which the pressure is stopped by means of an eccentric cam. The rotation of the eccentric cam effects the applying and stopping of pressure. After the pressure is stopped, an operator can remove jams and clean and repair mechanical parts around the rollers. Especially when a sheet is caught in the feed rollers, the applying of pressure must immediately be stopped. Otherwise, the applying torque will change suddenly.
However, the eccentric cam has a drawback in that it starts to rotate if it is stopped in an unstable posture. It is dangerous if the eccentric cam starts to rotate and adds pressure while an operator is engaged in cleaning or repair work. For example, in the case of press rollers having a diameter of 20 cm, a high pressure such as 2 tons is usually applied to both ends of the roller. When a sheet is passed through the rollers, the problem is that the pressure varies in the direction of the axis of the roller depending upon the sheet size (i.e. the widths of a sheet perpendicular to the feed direction). More specifically, when the sheet has a reduced size, it passes through the central part of the rollers so that the surfaces of the rollers are warped The warped roller surfaces cause an uneven distribution of pressure in the axial direction of the rollers. The uneven distribution of pressure results in uneven development and causes fatal wrinkles in the sheet.
Another problem is that the pressure varies per unit picture line in accordance with variations in the sheet sizes, thereby resulting in unstable color densities and poor quality of the pictures.